[css-regions] MapClass is removed from IDL spec

[foolip]

This issue is similar to #2639 but for https://drafts.csswg.org/css-regions/#the-namedflow-interface:

[Exposed=Window,
 MapClass=(CSSOMString, NamedFlow)] interface NamedFlowMap {
  NamedFlow? get(CSSOMString flowName);
  boolean has(CSSOMString flowName);
  NamedFlowMap set(CSSOMString flowName, NamedFlow flowValue);
  boolean delete(CSSOMString flowName);
};

The fix would be similar to 713ace7.